1. Field of the Invention
The present invention relates to a method of and a plant for combusting carbonaceous fuel in a fluidized bed by transporting oxygen to the combustion process by a solid oxygen carrier material. More particularly, the present invention relates to a power generation process where oxygen is fixed into a solid oxygen carrier material in a first fluidized bed and carbonaceous fuel is combusted in another fluidized bed by the oxygen released from the oxygen carrier material.
2. Description of the Related Art
Chemical looping combustion is a technology proposed for combusting fuels by transporting oxygen from combustion air to the fuel by means of an oxide compound acting as a solid oxygen carrier. The fuel is introduced in the so-called fuel reactor, or combustion reactor, and the fixing of oxygen from air to a suitable oxide compound is accomplished in another reactor, a so-called air reactor, or a regeneration reactor. The main advantage of using chemical looping combustion instead of conventional combustion in a single stage is that the CO2 produced in the combustion reactor is not diluted with nitrogen gas. Thus, the exhaust gas from the combustion reactor is, after separation of water, almost pure carbon dioxide, and does not require extra energy or costly external equipment for CO2 separation.
The development of chemical looping combustion, being new, is still in the pilot plant and the materials testing stage. Metal oxides, especially certain oxides of common transition-state metals, such as iron, nickel, copper and manganese have been suggested to be used as the oxygen carrier. The chemical looping principle may be applied either in a gas turbine cycle by having pressurized reactors for the fuel combustion and carrier regeneration, or in a steam turbine cycle with atmospheric pressure in the reactors.
The oxidation of the oxygen carrier with air in the air reactor is an exothermic reaction. Thus, a large amount of energy is to be recovered from the exhaust gas, oxygen-depleted air, discharged from the air reactor. Correspondingly, the reduction of the oxygen carrier in the combustion reactor is an endothermic reaction and consumes a considerable portion of the energy provided by the combustion of the fuel. While the decomposition of the oxygen carrying compound takes place only at a sufficiently high temperature, the combustion reactor also produces hot exhaust gas, mainly CO2 and water.
U.S. Pat. No. 5,447,024 discloses a method of generating power by chemical looping combustion, wherein hydrocarbon fuel is reacted with a metallic oxide in a fuel reactor to produce a first off gas containing carbon dioxide and water, and to reduce the metallic oxide to a reduced solid product. The reduced solid product is oxidized by air in an air reactor whereby a metal oxide, to be recycled to the fuel reactor, and a second off gas are produced. The first and second off gases are passed through first and second turbines, respectively, to produce power. Because of the chemical reaction between the fuel and the metallic oxide, the method is only suitable for liquid or gaseous fuels. Another drawback of the method is its complexity, especially because of the two turbines that are required to produce the power.
U.S. Pat. No. 6,572,761 discloses a chemical looping combustion process intended for combusting coal or biomass by using iron oxides as an oxygen carrier. According to the patent, the rate of addition of sulfur-containing fuel to the combustion bed is adjusted so as to minimize the formation of FeS, which might otherwise be transported to the air reactor. In this process it is, however, questionable whether the solid fuels can be efficiently oxidized with Fe2O3, except for the combustible volatile compounds released from the fuel.
It has also been suggested to use chemical looping combustion for combusting solid fuel by first gasifying the fuel in a separate gasifier. This, however, would require another reactor for the gasification, which would make the plant even more complicated. The additional gasification step would also generally reduce the efficiency and carbon conversion of the plant.
U.S. Pat. No. 6,143,203 discloses a process for partial oxidation of hydrocarbons, where a perovskite-type ceramic mixed conductor is circulated between an adsorption zone at an elevated temperature, for saturating the mixed conductor with oxygen, and a partial oxidation zone, for contacting the hot oxygen-saturated mixed conductor with a hydrocarbon. During the partial oxidation phase of the process, the sorbent reacts with the hydrocarbon, thereby producing hydrogen and carbon monoxide. For safety reasons, the oxygen-depleted mixed conductor removed from the partial oxidation is treated in a stripping section to remove residual unreacted hydrocarbon and/or partial oxidation reaction products prior to being returned to the adsorption unit.